The present invention relates to a copier, laser printer or similar image forming apparatus and, more particularly, to an image forming apparatus of the type having an intermediate image transfer member for transferring a black image and color images to a single sheet or similar transfer material one above the other.
A color image forming apparatus having an intermediate image transfer member implemented as, for example, a belt is disclosed in Japanese Patent Laid-Open Publication No. 5-11562 by way of example. In this type of apparatus, color toner images are sequentially formed on an image carrier and sequentially transferred to the belt one above the other, thereby forming a composite toner image on the belt. The composite toner image is transferred from the belt to a sheet or similar transfer material. The image transfer from the image carrier to the belt and the image transfer from the belt to the transfer material will be referred to as primary or belt transfer and secondary or sheet transfer, respectively. This kind of system has an excellent paper-free feature since the sheet does not wrap around the belt, compared to a system using a transfer drum. However, the belt must have a circumferential length guaranteeing at least the maximum print size. Moreover, the actual length of the belt is further increased in consideration of, for example, a period of time necessary for the return of a scanner. Such a belt increases the overall size and, therefore, the cost of the apparatus. In addition, for copies of small sizes, the period of time for one turn of the belt is excessively long, so that an additional copying time is needed even when only a single copy is desired.
In light of the above, there has been proposed a system which, by reducing the circumferential length of the belt, ensures a desired copying speed even with copies of small sizes and, in addition, prevents the allowable maximum print size from being reduced. Specifically, to produce a copy of large size approximate to the circumferential length of the belt, the system causes the belt to rotate without image transfer, i.e., to "idle" between the primary transfer of one color and that of another color, thereby guaranteeing, for example, an interval for the scanner to return.
However the system causing the belt to idle as mentioned above has some issues yet to be solved, as follows. Although no images are formed on the image carrier while the belt idles, the image carrier and belt are constantly held in contact. Hence, assuming a copy of large size, if an electric field for the primary image transfer is turned off, it is likely that a toner image is reversely transferred from the belt to the image carrier. Particularly, with an intermediate transfer belt having a medium resistance, the reserve transfer occurs easily even if the above-mentioned electric field is turned off. Specifically, potentials deposited on such a belt and the image carrier are about 0 V and about -700 V, respectively. Hence, although the toner on the belt is attracted due to the orientation of an electric field, such a degree of attraction cannot overcome the other forces including a mechanical force. If the electric field for image transfer is the same as the electric field for image formation, toner contaminating the background of the image carrier is transferred to the belt when the belt idles. Generally, since the background contamination of the image carrier cannot be fully avoided at the time of development, it is allowed within a certain range. However, if the transfer of the toner contaminating the background from the image carrier to the belt is allowed even during idling, the contamination is doubled, compared to copying using a sheet of small size and not involving idling.
Further, the idling scheme has a problem relating to the secondary transfer, i.e., the transfer from the belt to the sheet or similar transfer material. Usually, the belt has a medium resistance, i.e., a volume resistivity ranging from 1.times.10.sup.8 .OMEGA..cm to 10.sup.12 .OMEGA..cm (measured by JIS K6911). This kind of belt causes a potential deposited by primary transfer means to attenuate and then disappear due to the time constant thereof. Hence, it is possible to eliminate the need for AC corona discharger or similar means for discharging the belt, to obviate ozone particular to such discharging means, to reduce the cost, and to prevent the apparatus from increasing in size. Should the belt be made of an insulating material, means for discharging it would be necessary and would increase the size and cost of the apparatus, complicate control, and generate ozone.